August 2011
This report investigates methods used by inshore demersal longline vessels to increase the sink rates of lines to reduce the risk of interactions with seabird species. It also provides recommendations on ways to further increase those sink rates.

A survey was carried out to characterise the inshore bottom longline fishery on the southeast coast of the north island, and to discuss seabird interactions and mitigation measures with skippers. Vessels worked either an automatic or manual ‘clip on’ longline system. In both cases target fish species determined line configuration, specifically the sequence of floats and weights added to the mainline. Night-setting and tori lines were the most commonly employed mitigation measures, but were not used by all vessels or for all sets.

Time depth recorders (TDRs) were used to measure sink rates of bottom longlines on four observed vessels to quantify the availability of hooks to seabirds. For 3 vessels working ‘clip on’ gear, lines generally sank to 5 m within 60 s and 100 m behind the vessel and to 15 m within 120 s and 250 m behind the vessel. Sink times for one autoline vessel showed greater variation, with maximum sink times considerably longer than the ‘clip on’ vessels. Line set ups with multiple floats between weights, and those with the largest weight spacing produced the most variation in, and slowest, sink rates. The weight and flotation added to the line did not fully predict sink rates Setting speed and line tension helped to explain differences.

Recommendations on how to increase sink rate include the use of rope extensions for subsurface floats, and minimising line tension when possible. Further reducing the availability of hooks to seabirds may be possible by working with skippers to trial different weighting regimes to increase sink rates without adversely affecting fishing operations. Setting speed influenced how far behind the vessel TDRs sank to 5, 10 and 15 m depth and should be considered in conjunction with to tori line length.


Publication information

This report was commissioned by the Department of Conservation, Project MIT 2010/01

By D. Goad, Vita Maris


Conservation Services Programme
Department of Conservation
PO Box 10-420
Wellington 6143


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